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Jacking and Tunneling
In all jacking and tunneling operations, direction and distance are carefully
established prior to beginning the operation. The first step is the excavation
of jacking pits or shafts at each end of the proposed line. The pit
must be of sufficient size to provide ample working space for the jacking
head, jacks, jacking frame, reaction blocks, spoil removal and one or two
sections of pipe. Provisions are made for the erection of guide rails
in the bottom of the pit. For large pipe it is desirable to set the
rails in a concrete slab. If drainage is to be discharged from the
jacking pit, a collection sump and drainage pump are required.
Jacking Concrete Pipe
Concrete pipe is frequently installed by tunneling and jacking where deep
installations are necessary or where conventional open excavation and backfill
methods may not be feasible. Concrete pipelines were first jacked
in place by the Northern Pacific Railroad between 1896 and 1900. In
more recent years, this technique has been applied to sewer construction
with intermediate shafts along the line of the sewer as jacking stations.
The feasibility and planning of tunneling and jacking projects should be
coordinated with local concrete pipe manufactures and jacking contractors.
The usual construction sequence for tunneling and jacking concrete pipe is:
Excavation should not proceed the jacking
operation more than necessary. Material is trimmed so the tunnel bore slightly
exceeds the outside diameter of the pipe. Such a procedure results in minimal
disturbance to the adjacent soils. The
lead pipe is generally contained within a shield projecting from the mining
machine or equipped with a cutter or shoe to protect the pipe when excavating
by hand. Jacked
pipe tends to set or freeze when forward movement is interrupted, resulting
in significantly increased frictional resistance, therefore, continuous
operation is desirable. Occasionally a lubricant, such as bentonite slurry,
is pumped into the space between the tunnel bore and the outside of the
pipe to reduce frictional resistance. After the jacked pipe have reached
their final position, grout is frequently pumped into this same space
to insure continuous bearing with the surrounding soil.
Loads on Jacked Pipe
Two types of loads imposed upon concrete pipe installed by the jacking method are the axial load due to the jacking forces applied during installation and external earth and live loads.
The axial or thrust loads are transmitted from one concrete pipe section to another through the joint surfaces. To prevent localized stress concentrations, it is necessary to provide relatively uniform distribution of the axial loads around the periphery of the pipe. This is accomplished by assuring that the pipe ends are parallel within the tolerances prescribed by ASTM standards, using a cushion material, plywood, between the pipe sections, and care on the part of the contractor to insure that the jacking force is properly distributed through the jacking frame and parallel with the axis of the pipe.
The cross-sectional area of a standard concrete pipe wall is more than adequate to resist stresses encountered in normal jacking operations. For projects where extreme jacking pressures are anticipated due to long jacking distances or excessive unit frictional forces, intermediate jacking stations may be used, and greater care taken to avoid bearing stress concentrations.
The major factors influencing the vertical earth load on pipe installed by jacking are:
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